ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-5-77-2005Heterogeneous conversion of NO<sub>2</sub> and NO on HNO<sub>3</sub> treated soot surfaces: atmospheric implicationsKleffmannJ.1WiesenP.11Physikalische Chemie/FB C, Bergische Universität Wuppertal, D-42097 Wuppertal, Germany20012005517783This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.This article is available from http://www.atmos-chem-phys.net/5/77/2005/acp-5-77-2005.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/5/77/2005/acp-5-77-2005.pdf

In the present study, the heterogeneous conversion of nitrogen oxide (NO)
and nitrogen dioxide (NO<sub>2</sub>) was studied at atmospheric humidity levels
on flame soot surfaces treated with gaseous nitric acid (HNO<sub>3</sub>).
In addition, the heterogeneous reaction of HNO<sub>3</sub> on soot
was investigated at atmospheric humidity.
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For the treatment of soot by pure HNO<sub>3</sub> only reversible uptake with a
surface coverage of ~1-2x10<sup>14</sup> HNO<sub>3</sub> cm<sup>-2</sup> was
observed for HNO<sub>3</sub> mixing ratios in the range 250-800ppbv. Only
for higher HNO<sub>3</sub> mixing ratios of &gt;800ppbv the formation of NO and
NO<sub>2</sub> was observed. The results were not affected by the addition of NO.
In none of the experiments with HNO<sub>3</sub> the formation of nitrous acid
(HONO) was observed. For HNO<sub>3</sub> mixing ratios &lt;600ppbv the upper limit
yields for HONO, NO<sub>2</sub> and NO were found to be &lt;0.2%, &lt;0.5%
and &lt;1%, respectively. Compared to untreated soot, the product
formation of the reaction of NO<sub>2</sub> with soot was not significantly
affected when the soot surface was treated with gaseous HNO<sub>3</sub> prior to
the experiment. Only for high surface coverage of HNO<sub>3</sub> the formation of
HONO was suppressed in the initial phase of the reaction, probably caused by
the blocking of active sites by adsorbed HNO<sub>3</sub>.
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Under the assumption that the experimental findings for the used model flame
soot can be extrapolated to atmospheric soot particles, the results show
that the reactions of HNO<sub>3</sub> and HNO<sub>3</sub>+NO on soot surfaces are
unimportant for a "renoxification" of the atmosphere and do not represent
an atmospheric HONO source. In addition, the integrated HONO yield of ca.
10<sup>14</sup>cm<sup>-2</sup> in the reaction of NO<sub>2</sub> with soot is not
significantly influenced by simulated atmospheric processing of the soot
surface by HNO<sub>3</sub>, and is still too small to explain HONO formation in
the atmosphere.